The CPRI link capacities required between BBU and RRH in the C-RAN architecture are as follows:

Table 1. CPRI data rates in function of radio technologies

In case of a network with LTE Carrier BW of 20MHz and 2x2 MIMO antenna configuration, the maximum IP throughput in the cell is only 150Mbps. So theoretically, a backhaul network with 150 Mbps should be able to handle the traffic. In reality, however, a CPRI link between BBU (at CO) and RRH (at Cell Site) requires a bandwidth as high as 2.4576Gbps. Why such a high bandwidth is needed?

Let's see how data traffic is delivered in a standalone base station, where BBU and RRH are kept together, as seen in Figure (a) below (Please note, in the 2X2 MIMO configuration illustration below, only one (instead of two) antenna is shown for the sake of simplicity).

In the LTE Baseband PHY layer, OFDM symbol waveforms (I waveform and Q waveform) are generated once all the signal processing, such as channel coding, modulation, IFFT(inverse FFT), are completed. Through sampling these two waveforms, you get a set of digital sample data (I sample 15 bits and Q sample 15 bits), which then are sent to their associated DAC (Digital to Analog Converter), and converted into analog I/Q signals. Thereafter, once RF processing (modulation/mixing/power amplification) is performed, the radio signal is finally radiated over the air through the antenna.

During the sampling process of the OFDM symbol waveforms, the sampling frequency was 30.72MHz (when LTE carrier BW is 20MHz), and the sampling bit-width for I and Q samples was 15 bits (LTE case). Therefore, the resulting I/Q sample data rate was 0.9216Gbps (=30bit/30.72Mhz).

The massive I/Q samples generated this way are delivered to DAC through the built-in internal digital bus in the base station system.

Unlike the standalone base station, in C-RAN, BBUand RRH are kept separated from each other, and connected through the CPRI interface. Because radio parts are located in RRH, baseband I/Q samples must be delivered to DAC in RRH. As you can see in the Figure (b) below, I/Q sample data (0.9216Gbps) are carried in the payload of a CPRI frame. These sample data and the control word added to them form a CPRI frame (0.983Gbps=16/15*0.9216Gbps). Then, after 8B/10B encoding process, the frame is forwarded through the CPRI link (1.2288Gbps = 10/8*0.983Gbps) finally to RRH.

Now we know as each antenna requires 1.2288Gbps, 2X2 configuration with two antennas requires the double CPRI link capacity of 2.4576Gbps.

In summary, in a standalone base station, I/Q samples can be simply delivered through the built-in internal digital bus to DAC. However, in C-RAN, I/Q samples are generated at BBU in CO, and thus must travel far to reach DAC at RRH in a cell site. And this requires a CPRI link with higher bandwidth.

Let's assume a network with LTE Carrier BW of 20 MHz, 2x2 antenna configuration, and three RRHs (3-sector) in a cell site. If we do math, each RRH will require the CPRI capacity of 2.4576Gbps, and thus each cell site will need the capacity of 7.3728Gbps.

2.4576 Gbps/RRH (20MHz, 2x2) x 3 RRHs = 7.3728Gbps

Then a network with two bands will require:

2.4576 Gbps/RRH (20MHz, 2x2) x 3 RRHs x 2 band = 14.7456Gbps

In this case, if each RRH uses one fiber link, each cell site would need six fiber links, which can be very costly.

As an effort to lower the fiber cost, now WDM equipment is installed between a CO and cell site, assigning different wavelengths to each RRH.

Delivering compressed I/Q samples, another possible way of lowering the fiber cost, will be introduced in the next post.